KDEnLive 0.7.5 was released a few days ago, so I installed it on my Ubuntu machine to see how the app has been progressed since the last time I tried it. KDEnLive is the most actively developed, and easiest to use Linux video editor. Its UI resembles Sony Vegas in many ways; if you know how to use Vegas, you won't have trouble with KDEnLive.

It is possible to override the detected aspect ratio by double-clicking the clip in the Project Tree (clip bin) to open the clip properties dialog. Then, you can make the change in the Advanced tab. It is true that currently you can not override the detection of interlace vs. progressive. I recently added that in MLT, so we can very easily add that to the next Kdenlive release.

It should be possible to preview transitions without rendering. Maybe something went wrong and broke that for you. In any case, it will perform frame-dropping if you play, and you might not get much throughput depending on resolution, cpu, etc. However, you should also be able to step and scrub it to get an idea. Finally, the preview monitor provides an option to turn off the frame-dropping.

I would like to add that in addition to Video4Linux capture there is also Firewire (DV and HDV with preview) and screen capture.

I also think one of the more interesting features is the online sharing of transition wipes, project settings, and render profiles. Within the tool, you can browse, download, install, and start using them.